1998 GMC Jimmy SLT warm air only from heating ducts
When the vehicle is fully warmed up (coolant temp about 200Fper vehicle gauge) the air temperature from all the ducts is only warn...not hot. I have replaced the stat even thought the temp gauge indicate normal operating temperature. I have removed the the lines from the heater core and back flushed the heater core.I had very good flow throught the core. The inlet line to the core is very hot to the touch and the out let line is warn so it seem like the system is exchanging heat. All the air positions fubction properly. By this I mean in floor mode you only get air fron the floor duct. In I/P (instrument pannel) mode it blows from the I/p only and in defrost mode it only blows on the windshield.
Thanks for you time.
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There are a couple of possibilities. If you have a temp gauge in the car, is it showing normal temperature when the car is fully warmed up? If not, you could have a thermostat problem. If you don't have a temp gauge, then check the temp of the upper radiator hose with the engine fully warmed up. If it is not very warn to the touch, check the thermostat. Assuming that the thermostat is not stuck wide open, and the vehicle is at normal temperature, then the most likely causes are either a stuck blend door in the heater control box, or a linkage/vacuum issue with the heater control to the control box.
This sounds like a thermostat that is fully open all the time. It allows the engine coolant to warm when sitting still (no air being forced thru the radiator), but even that goes away when moving, since the air through the radiator will remove that heat from the coolant. Does you van have a temp gauge? Is so, at full operating temp. most vehicles have coolant temps of 180-195 degrees F, depending on the thermostat installed. Anything less indicates a thermostat that is starting to malfunction.
Air can be purged. While your engine is cool remove radiator and reservoir caps. Place funnel in radiator. Turn heat to the highest position and start vehicle. Add coolant to radiator as needed while vehicle warms up. You should see bubbles coming out of radiator. Fill reservoir to between cold and hot fill lines. Replace caps when bubbling stops but before coolant temperature reaches normal operating temperature. Never remove the radiator cap until engine has fully cooled down.
When the coolant flush was done, did you have the thermostat changed as well? I would have the thermostat replaced. If the temperature gauge was reading properly, but you still had warm, not hot air, it would lean more towards the heater core being clogged, but that deosn't seem to be the issue your having, so i would go with the thermostat replacement. And whereever you have the repairs done at...make sure they fully bleed all the air out of the cooling system, this could also cause poor warm-up / heating problems. Hope this helps...
How do you know it gets hot? Are you going by the temperature gauge on the dash? Or have you checked under the hood and found out the engine really is running hot? ...Reason I ask is, something just doesn't sound right in your description. After just 5 minutes from starting and driving away, the engine would not yet be at normal operating temperature. That would explain the heater not yet warm. But the temp. gauge says hot? If so, I would replace the temperature sending unit on the engine. the one for the dash gauge, and see if that was the problem-not reading the temp. accurately. Couple of other things: Are you sure you got all the air out of the coolant system? Start the car with the radiator cap off and let it run until the engine is warmed up fully, then shut it off and install the cap. Does your reservoir tank stay about half full where it should be, between the full-hot and full-cold marks? If your problem persists. I would have the coolant system pressure tested, and see if you could get a true temperature reading of the coolant when it seems to be running hot. Good luck.
Possibly the intake air temperature sensor or the engine coolant temp sensor are stuck at full hot. The computer will schedule too little fuel--especially with the engine cold and you'll have very little power until the engine actually gets closer in temp to the stuck sensor. The engine coolant temp sensor sends signals to the computer NOT to your temperature gauge on the instrument panel. That is a different sender. So your coolant temp gauge on the instrument panel may show normal.
If I understand you correctly, your temp gauge shows correct operating temperature, your blower fan blows air that you can feel with your hand from the right floor or dash outlet, but that air doesn't feel warm until you are moving?
If so, several possibilities come to mind. One is that your coolant level is low. Even though your temp gauge reads in the right range, the coolant level could be low enough that it doesn't reach the heater core until there is enough engine RPM to create more pressure from the water pump and force what coolant is there toward the heater core. Another possibility is that your heater controls are not able to do their job. A stuck flapper in the duct or a malfunction with the temperature control system could be causing the problem, and at 15-20 MPH there is enough air coming through the outside air vents to provide some heat.
I'd check the coolant level first, though. The other possibilities are more far-fetched.